Nitric oxide analysis

ABSTRACT

Method and apparatus for determining the nitric oxide content of a sample gas stream. The nitric oxide in the gas stream is reacted with an excess amount of ozone so as to completely convert the nitric oxide into nitrogen dioxide. The resulting gas containing nitrogen dioxide and residual ozone is passed through a scrubber which removes the ozone and does not affect the nitrogen dioxide content of the gas. The resulting gas is then analyzed for its NO2 content, which is a function of the nitric oxide content of the sample gas. The method and apparatus may also be utilized for determining the total amount of nitrogen dioxide and nitric oxide in a sample gas. The invention is particularly applicable to the monitoring of air for air pollution control.

United States Patent [151 3,652,227 Harman, III et al. [451 Mar. 28,1972 i541 NITRIC OXIDE ANALYSIS [72] Inventors: John N. Harman, III,Placentia; Radhak- Pnmary Drummer-Joseph Scovronek Assistant Examiner-R.M. Reese Brea both of Attorney-Robert J. Steinmeyer and William F.McDonald [73] Assignee: Beckman Instruments, Inc., Fullerton,

Calif. I [57] ABSTRACT Filed? 1969 7 Method and apparatus fordetermining the nitric oxide con- [211 No: 885,007 tent of a sample gasstream. The nitric oxide in the gas stream is reacted with an excessamount of ozone so as to completely convert the nitric oxide intonitrogen dioxide. The resulting U-s. R, S, V containing nitrogen dioxideand residual ozone is passed 23/253 23/254 252/463 through a scrubberwhich removes the ozone and does not at- [5 I] "Bold 4 Colb 21/46 31/10fect the nitrogen dioxide content of the gas. The resulting gas OISearch ..23/232, 253, 2 S, is then analyzed f its N02 content, which i afunction of the nitric oxide content of the sample gas. The method andap- [56] References cued paratus may also be utilized for determiningthe total amount OTHER PUBLICATIONS of nitrogen dioxide and nitric oxidein a sample gas. The inventlon is particularly applicable to themonitoring of anfor Spanily, V., Chem. AbStr. 50, f. ai pollufion onn- LGanz et al., et al., Chem. Abstr. 57, 3245 f. (1962) Rossano et al.,Chem. Abstr. 60, 4753 f. 1964) 24 Claims, 2 Drawing Figures 46/ACTIVATED CHARCOAL AIR PAn-jmfumza 1972 3, 652.227

NR SCRUBBER N02 2 7 CHAMBER \2 ANALYZER i OZONE SOURCE AIR SCRUBBER r MREACTION +03 0 N02 No CHAMBER SCRUBBER L ANALYZER F as 4o 42 REACTIONCHAMBER 46 ACTIVATED CHARCOAL INVENTORS R. M. NETI ATTORNEY NITRIC OXIDEANALYSIS CROSS-REFERENCE TO RELATED APPLICATIONS This application isrelated to or embodies some of the features disclosed in our copendingapplication entitled No, Analysis and Scrubber Therefor, Ser. No.885,006, filed concurrently herewith, and copending application of R. M.Neti entitled Scrubber Apparatus, Ser. No. 747,721, filed July 25, 1968,now US. Pat. No. 3,579,305 issued May 18, 1971, both of whichapplications are assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION This invention relates generally to gasanalysis and, more particuarly, to an improved method and apparatus fordetermining the nitric oxide content of a gas sample stream. Theinvention is also applicable to the determination of the total amount ofnitric oxide and nitrogen dioxide in a gas.

Nitric oxide exists in the atmosphere in conjunction with nitrogendioxide as common air pollutants. In order that dangerous levels ofthese constituents may be known and corrective measures may be taken,means is required for practically and inexpensively deten'nining thelevel of these constituents in air. The present invention is directedprimarily to the determination of nitric oxide in air, but is alsoapplicable to the determination of the total nitric oxide and nitrogendioxide content of air. While the description of the present inventionis directed primarily to the determination of these constituents in air,it is to be understood that the invention is also applicable to thedetermination of such constituents in other gaseous media.

The conventional analytical technique for the determination of nitricoxide is called the Saltzman method and is described in detail in UnitedStates Public Health Service Publication No. 999-AP-11. In the Saltzmanmethod, nitric oxide in air is converted into an equivalent amount ofnitrogen dioxide by passage of the air through a permanganate bubblerwhich chemically oxidizes the nitric oxide. A colorimetric analysis isutilized to determine the NO, content of the resulting gas. Thisanalysis involves the absorbing of the N in a suitable absorbingreagent. Within about fifteen minutes a red-violet color appears in thereagent which may be read visually by comparing with suitable colorstandards or transferred to a cuvette and read by an appropriateinstrument such as a spectrophotometer. In similar methods ofdetermining nitric oxide in air, K 0 0, is employed as the oxidizingsolution for the conversion of NO to N0 or the sample air stream ispassed through an oxidant impregnated medium, such as a filterimpregnated with CrO While the colorimetric method of analyzing N0 isquite satisfactory, the aforementioned techniques for converting nitricoxide into N0 suffer from several deficiencies. As stated in theaforementioned Public Health Service Publication, the conversionefficiency of nitric oxide to nitrogen dioxide by a permanganate bubblermay be commonly as low as 70 percent. The other methods mentioned abovefor converting nitric oxide to nitrogen dioxide also suffer from lessthan 100 percent conversion efficiency of nitric oxide to nitrogendioxide. In addition, the aforementioned oxidizer mediums have limitedlives, on the order of only about one week, while functioning at theirmaximum conversion efficiencies. Also, when using an oxidizing solution,there is the possibility that some of the solution may be carried overinto the absorbing reagent utilized in the colorimetric analyticalprocess, thus resulting in errors in the determination of N0 Theaforementioned oxidation techniques further suffer from the disadvantagethat the conversion efficiency of NO to N0 depends upon the level ofhumidity in the air sample. Thus the determination of nitric oxide inair by the aforementioned methods is dependent upon the humidity contentof the air. In addition, the aforementioned oxidizer mediums oftenrequire complex sample handling equipment. Therefore, what is needed isan improved method and apparatus for determining the nitric oxidecontent of air which will avoid some if not all of the aforementioneddeficiencies in the conventional techniques for determining nitricoxide.

SUMMARY OF THE INVENTION The principal object of the present inventionis to provide an improved method and apparatus for determining nitricoxide in a sample gas stream.

Another object of the invention is to provide a method and apparatus fordetermining the total amount of nitric oxide and nitrogen dioxide in agas sample.

According to the principal aspect of the present invention, nitric oxidein air or other sample gas is reacted with at least the stoichiometricamount, preferably an excess amount, of ozone so as to completelyconvert the nitric oxide into nitrogen dioxide. The resulting gas streamcontaining nitrogen dioxide and residual ozone is then passed through ascrubber which removes the ozone from the gas but does not affect thenitrogen dioxide content thereof. Thereafter, the nitrogen dioxidecontent of the gas is determined as a measure of the nitric oxidecontent of the initial air stream. If the sample contains nitrogendioxide as well as nitric oxide, the aforementioned method and apparatuswill provide a measure of the total nitric oxide and nitrogen dioxidecontent of the air stream. If the air stream is to be monitored for itsnitric oxide content alone and it contains NO, and species which willreact with ozone to produce interferents to which the N0 analyzer issensitive, the air stream is initially passed through a scrubber whichwill remove NO and such species prior to reacting the air stream withozone. The method and apparatus of the present invention have theadvantage that they are simple, inexpensive and permit the rapid andcontinuous monitoring of the NO or NO plus N0 content of an air stream.Moreover, by the use of ozone as the oxidant in the present inventionfor converting NO into N0 most if not all of the deficiencies present inthe aforementioned prior art techniques of nitric oxide analysis areovercome.

Other objects, aspects and advantages of the invention will become moreapparent from the following description taken in connection with theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic view of asimplified form of the invention utilized for determining either NOalone or in combination with N0 and FIG. 2 is a schematic view of thepreferred form of the invention utilized for monitoring the NO contentof air.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 indetail, there is illustrated a simplified form of the apparatus of thepresent invention, generally designated 10. The apparatus 10 includes areaction chamber 12, ozone source 14, scrubber 16 and nitrogen dioxideanalyzer 18. The chamber 12 has a sample inlet 20 for continuouslyconveying air or other sample gas into the chamber. The ozone source isconnected to the reaction chamber by means of a conduit 21. The ozonesource may comprise a pressurized tank containing ozone which may bebled into the reaction chamber via the conduit 21. The source 14 mayalso comprise means for generating ozone in situ such as will bedescribed later in connection with FIG. 2. The scrubber I6 is connectedto the reaction chamber 12 by means of a conduit 22 while the N0analyzer 18 is connected to the scrubber by a conduit 24. Thus, an airstream entering the inlet 20 will pass in sequence through the reactionchamber 12, scrubber l6 and NO, analyzer 18 and will exit therefrom viaa vent 26.

The scrubber 16 contains a material which is capable of v scrubbing orremovingozone and other species contained in a sample air stream whichwill interfere with the analysis of NO by the analyzer 18, yet will notaffect the N0 content of the gas introduced into the scrubber. Such asuitable material is argentic oxide (AgO) which is capable of scrubbingozone.

S0,, H 8, halogen gases, mercaptans and other organic sulfur .compoundswith better than 90 percent efiiciency yet does not affect the NO,content of an air stream passing therethrough. lf air is the sample gas,the scrubber should contain less than 2 grams AgO and preferably about 1gram to prevent delay in the transmission of NO, therethrough.Preferably the AgO is in the form of small granules. The AgO scrubbermaterial and its characteristics are described in greater detail in ouraforementioned copending application entitled NO, Analysis and ScrubberTherefor.

The N analyzer 18 may comprise a colorimetric instrument such asutilized in the aforementioned Saltzman method described in PublicHealth Service Publication No. 999-AP- l l, and thus will include anabsorbing reagent and an instrument for determining color change, suchas a spectrophotometer. Preferably, however, the NO, analyzer is acoulometric galvanic cell of the type disclosed in US. Pat. No.3,314,864 to Hersch. This cell contains a body of aqueous halideelectrolyte in which there is immersed a cathode of inert conductivematerial, such as platinum or graphite, and an anode of either activecarbon, calomel or silver chloride. If air containing NO, is introducedinto the cell, it will dissolve in the electrolyte and the followingreactions will take place if a potassium iodide electrolyte is employed:

Thus, the NO, in the air sample oxidizes the iodide ions in theelectrolyte to form iodine. The iodine bearing electrolyte is passedover the cathode in the cell where the iodine is reduced back to iodideions. A galvanic current is generated by the electrode pair in the cellwhich is a function of the rate of entry of the NO, into the cellelectrolyte, and thus a measure of the level of the NO, in the gas beingintroduced into the cell. Preferably a current amplifier and meter whichmay be used in conjunction with a recorder, not shown, is coupled to theelectrodes of the l-lersch cell to provide a continuous visualindication of the current output of the cell.

The apparatus may be utilized for determining the nitric oxide contentof air alone if the air sample does not contain NO, or certain specieswhich will react with ozone to produce interferents that are not removedby the scrubber l6 and thus would interfere in the analysis of NO, bythe analyzer 18. Such a species is SO, which may react with ozone toform 80;, that will pass through the scrubber 16 and interfere in theanalysis performed by the NO, analyzer 18. To determine the nitric oxidecontent of an air stream which is free of such a species and NO,, theair stream is continuously conveyed directly into the reaction chamber12 via the inlet 20. In the chamber 12 the nitric oxide will react withozone which is bled into the chamber from the source 14. The ozoneconcentration must be the stoichiometric amount, and preferably anexcess amount, required to completely convert the nitric oxide in thesample air stream to nitrogen dioxide. The reaction between ozone andnitric oxide occurring in the chamber 12 may be represented as follows:

The effluent from the reaction chamber 12 containing an equivalentamount of NO,, residual ozone and interferents contained in the samplegas stream is conveyed to the scrubber 16 via the conduit 22. The silveroxide in the scrubber removes the common interferents found in air aswell as ozone but does not react or absorb NO, in the gas stream. Thus,the scrubber l6 permits the quantitative transmission of NO, in the gasstream into the analyzer 18 through the conduit 24, and the air streamexits from the analyzer via the vent 26. The analyzer 18 determines theN0 content of the efiluent from the scrubber 16 and thus provides ameasure'of the nitric oxide content of the sample air stream introducedinto the apparatus 10.

It can be appreciated that is the air sample contains nitrogen dioxideas well as nitric oxide, but no species such as S0, which will reactwith ozone to form interfering constituents that cannot be removed bythe scrubber 16, the apparatus 10 may be employed for determining thetotal amount of nitric oxide and nitrogen dioxide in an air stream. lnthis case, the NO, in the sample air stream will be unaffected bypassage through the reaction chamber 12 with the result that the totalamount of nitrogen dioxide in the effluent from the chamber 12 will beequivalent to both the nitric oxide in the sample air stream, which isconverted to nitrogen dioxide by the ozone in the reaction chamber, andthe initial nitrogen dioxide in the sample air stream. Since the silveroxide scrubber does'not affect N0 the output signal of the analyzer 18provides a measure of the total amount of nitric oxide and nitrogendioxide in the sample air stream.

The preferred form of the apparatus of our invention is illustrated inFIG. 2 and is generally designated 30. The apparatus 30 is utilized fordetermining only the nitric oxide content of an air or other gas stream,and has the advantage over the apparatus 10 in that it is not subject toerrors due to interfering species such as S0,.

The apparatus 30 is similar to apparatus 10 in that it employs areaction chamber 32, ozone scrubber 34 and NO, analyzer 36. The analyzermay be either one of the types discussed above with respect to analyzer18. The scrubber 34 is connected to the chamber 32 by means of a conduit38 while the analyzer 36 is connected to the scrubber 34 by a conduit40. An outlet 42 is provided on the analyzer 36 for venting the samplestream to atmosphere after analysis thereof.

The ozone source 44 in the apparatus 30 may be the same as the source 14utilized in apparatus 10, but preferably comprises an apparatus forgenerating ozone in situ. Such an apparatus includes a scrubber 46containing a scrubbing material such as activated charcoal which iscapable of absorbing all interferents from an air stream so that clean"air, i.e., a mixture of nitrogen and oxygen, will be emitted from thescrubber. The scrubber 46 is provided with an inlet 47 for introducingair therein. The source 44 also includes a reaction chamber 48 which isconnected to the scrubber 46 by means of a conduit 50. The chamber 48contains means for continuously converting oxygen into ozone. This maycomprise a source of ultraviolet radiation for photo-oxidizing oxygen toozone or a pair of electrodes for generating an electric dischargesufficient to convert the oxygen into ozone. A suitable source ofradiation for practicing the present invention is a double bore quartzlamp sold under the trade name Pen-ray Lamp, Model No. l 1 SC-l marketedby Ultra-violet Products Inc. of San Gabriel, California. Obviously anyother source of radiation may be employed if it is capable ofphoto-oxidizing oxygen to ozone. Electric discharge techniques forconverting oxygen to ozone'are well known in the art and need not bedescribed in detail here. The ozone forming reaction chamber 48 isconnected to the reaction chamber 32 by a conduit 52.

The apparatus 30 includes an additional scrubber 54 which is connectedto the reaction chamber 32 by a conduit 56 and includes a sample inlet58. The scrubber 54 contains a material which removes all species in agas stream which will interfere in the analysis downstream of thescrubber but will not affect the nitric oxide content of the sample airstream. Such a suitable scrubbing material is either Ca[0l-l] or KOH,each of which is capable of efficiently scrubbing S0,, halogens, NO,, H8, and mercaptans. These scrubber materials and their characteristicsare described in greater detail in the aforementioned Neti application.These materials do not scrub ozone. However, this is of no consequencein the instant invention inasmuch as ozone does not interfere in theanalysis process since it is ultimately removed by the scrubber 34 fromthe gas conveyed through the apparatus.

Since the scrubber 54 removes interferents from the sample gas, thescrubber 34, unlike scrubber 16, need only to remove ozone andquantitatively pass N0 The scrubber 34 may be identical to the scrubber16, that is, contain AgO which scrubs ozone but does not affect the NO,content of a gas stream passing therethrough. Alternatively, the ozonein the gas stream entering the scrubber 34 may be removed by thermalanalyzer 36 were a colorimetric instrument, the scrubbing of 5 ozone bythermal decomposition would have the possible disadvantage that some N0in the gas passing through the scrubber may be converted to di-nitrogenpentoxide which would affect the colorimetric analysis. However, withthe use of the aforementioned Hersch galvanic cell, scrubbing by meansof either the use of silver oxide or thermal decomposition may beutilized without any adverse affects on the N0 analysis of the effluentemanating from scrubber 34.

In operation of the apparatus 30, ambient air is continuously conveyedthrough inlet 47 into the scrubber 46-. The clean" air from the scrubber46 passes into the reaction chamber 48 where the oxygen in the airstream is converted to ozone. It is to be understood that the meansemployed for converting oxygen to ozone in reaction chamber 48 mustproduce at least the stoichiometric amount, and preferably an excessamount, necessary to completely react with the nitric oxide in the airstream being analyzed. The ozone produced in the reaction chamber 48 isconveyed by the conduit 52 into the chamber 32. The sample air streambeing monitored for its nitric oxide content is introduced into thescrubber 54 via the inlet 58. The scrubber 54 will appreciably removeall interferents from the sample air stream, including N0 but will passnitric oxide quantitatively and ozone if it is present in the sample airstream. The nitric oxide bearing effluent from the scrubber 54 will passinto the chamber 32 through the conduit 56. In the chamber 32 the nitricoxide in the air will react with ozone produced in the chamber 48 toproduce an equivalent amount of nitrogen dioxide. The effluent from thechamber 32 will therefore contain nitrogen dioxide and residual ozone,but no other interferents as is the case in the apparatus 10. Theeffluent from chamber 32 is then conveyed by the conduit 38 into thescrubber 34 which removes the ozone and thereafter the effluent fromscrubber 34 passes through conduit 40 to the N0 analyzer 36. Theanalyzer 36 determines the N0 content of the gas conveyed thereto andtherefore provides a measure of the nitric oxide content of the sampleair stream initially introduced into the apparatus.

An apparatus as shown in FIG. 2 employing a Model No. l 1 SC-l Pen-rayLamp for generation of ozone, calcium hydroxide as the initial airscrubber 54, and silver oxide as the ozone scrubber 34, has beensuccessfully employed in both colorimetric and galvanic nitric oxideanalyses with the result that the analyses compare quite favorably withthe standard Saltzman analytical technique referred to above. Ourapparatus, however, has the advantage over the Saltzman technique inthat almost 100 percent conversion of NO to N0 is achieved by the use ofthe ozone oxidation technique, and there is no possibility of solutioncarryover from an oxidizing solution into the gas sample train. Also,the analysis is not dependent upon the level of humidity of the sampleair being analyzed. Most importantly, when the ozone is generated insitu as by the apparatus 44 using a Pen-ray lamp, for example, there isan unlimited source of the oxidant available to efficiently convert NOto N0 over a very long period, on the order of 50,000 hours or more, incontrast to the conventional oxidation mediums which remain efiicientfor only about 1 week. The invention further has the advantage that itrequires no complex sample handling equipment, little maintenance andskill to operate, and permits fast and continuous monitoring of thenitric oxide content of a gas sample stream.

Although we have herein shown and described our invention in what wehave conceived to be the most practical and preferred embodiments, it isrecognized that departures may be made therefrom within the scope of ourinvention, which is not to be limited to the details disclosed hereinbut is to be accorded the full scope of the claims so as to embrace anyand all equivalent structures, methods and materials.

What is claimed is:

l. A method of determining nitric oxide in a sample gas streamcomprising the steps of:

providing scrubber means capable of scrubbing at least ozone from a gasstream containing nitrogen dioxide without appreciably affecting thenitrogen dioxide content of the gas stream; reacting the nitric oxide inthe sample gas stream with at least the stoichiometric amount of ozoneso as to completely convert the nitric oxide into nitrogen dioxide;

conveying the resulting nitrogen dioxide containing gas stream to saidscrubber means; and

thereafter determining the nitrogen dioxide content of the gas streamemanating from the scrubber means as a measure of the nitric oxidecontent of said sample gas stream.

2. A method as set forth in claim 1 wherein:

the nitric oxide in said gas stream is reacted with an excess amount ofozone so that said resulting gas stream will contain ozone as well asnitrogen dioxide.

3. A method as set forth in claim I wherein:

said ozone is produced by reacting ultraviolet radiation with an oxygenbearing gas.

4. A method as set forth in claim 1 wherein:

said ozone is produced by generating an electric discharge in an oxygenbearing gas.

5. A method as set forth in claim 1 wherein:

said scrubber means contains AgO.

6. A method as set forth in claim 1 wherein said sample gas streamcontains interferents other than ozone and including the additionalsteps of:

providing second scrubber means capable of scrubbing said interferentsfrom said sample gas stream without appreciably affecting the nitricoxide content thereof; and passing said sample gas stream through saidsecond scrubber means prior to reacting the stoichiometric amount ofozone with the nitric oxide in the sample gas stream.

7. A method as set forth in claim 6 wherein said second scrubber meanscontains a material selected from the group consisting of Ca(OI-l andKOH.

8. A method as set forth in claim 6 wherein said first-mentionedscrubber means contains AgO.

9. A method as set forth in claim 6 wherein said first-mentionedscrubber means scrubs ozone by thermal decomposition thereof.

10. A method of continuously monitoring the nitric oxide content of anair sample stream containing interferents comprising the steps of:

scrubbing the interferents from the air sample stream;

reacting the nitric oxide in the air sample stream with an excess amountof ozone so as to completely convert the nitric oxide to nitrogendioxide and also provide ozone in the resulting gas stream;

scrubbing the ozone from said resulting gas stream; and

thereafter determining the nitrogen dioxide content of the scrubbed gasstream as a measure of the nitric oxide content of said air samplestream.

11. A method as set forth in claim 10 wherein said ozone is scrubbedfrom said resulting gas stream by passing the stream over Ago.

12. A method as set forth in claim 1 1 wherein:

said interferents are scrubbed by passing the air sample stream over amaterial selected from the group consisting of Ca(OH) and KOH.

13. A method as set forth in claim 11 wherein said excess amount ofozone is provided by:

passing a second air stream through an activated charcoal scrubber sothat interferents are absorbed and a mixture consisting essentially ofnitrogen and oxygen is emitted from the scrubber as an effluent; and

reacting the oxygen bearing effluent from the activated charcoalscrubber with ultraviolet radiation.

14. A method of determining the total nitric oxide and nitrogen dioxidecontent of a sample gas stream containing interferents comprising thesteps of:

reacting the nitric oxide in the sample gas stream with at least thestoichiometric amount of ozone so as to completely convert the nitricoxide into nitrogen dioxide;

passing the resulting gas stream through a scrubber containing AgO toscrub said interferents and any ozone present in the gas stream; and

thereafter determining the nitrogen dioxide content of the effluentemanating from said scrubber whereby the total nitric oxide and nitrogendioxide content of the sample gas stream is determined. 15. An apparatusfor determining the nitric oxide content of a sample gas streamcomprising:

means for providing a supply of ozone; a reaction chamber; means forconveying ozone from said first-mentioned means and the sample gasstream to said reaction chamber whereby the nitric oxide in said samplegas stream will be converted into nitrogen dioxide in the chamber sothat a nitrogen dioxide and ozone containing gas stream is emitted fromthe reaction chamber as an effluent;

scrubber means for scrubbing ozone from said gas stream withoutappreciably affecting the nitrogen dioxide content of the gas stream sothat a nitrogen dioxide containing gas stream is emitted from thescrubber means as an effluent;

means for conveying the effluent from said reaction chamber to saidscrubber means;

analyzer means for determining the nitrogen dioxide content of said gasstream; and

means for conveying the effluent from said scrubber means to saidanalyzer means.

16. An apparatus as set forth in claim wherein said scrubber meanscontains AgO as a scrubber material.

17. An apparatus as set forth in claim 15 wherein said scrubber meanscontains means capable of thermally decomposing ozone thereby scrubbingthe ozone from said gas stream.

18. An apparatus as set forth in claim 15 wherein said means forproviding a supply of ozone includes a source of ultraviolet radiationfor photo-oxidizing oxygen to ozone.

19. An apparatus as set forth in claim 15 wherein said means forproviding a supply of ozone includes means for generating an electricdischarge sufficient to convert oxygen into ozone.

20. An apparatus as set forth in claim 15 including:

second scrubber means capable of scrubbing interferents from the samplegas stream without appreciably affecting the nitric oxide contentthereof so that a nitric oxide containing gas stream is emitted from thesecond scrubber means as an effluent; and

said sample gas conveying means includes means for introducing thesample gas stream into said second scrubber means and means fordelivering the effluent from said second scrubber means to said reactionchamber.

21. An apparatus as set-forth in claim 20 wherein said second scrubbermeans contains a material selected from the group consisting of Ca(Ol-land KOH as a scrubber material.

22. In an apparatus for continuously monitoring the nitric oxide contentof a sample air stream containing interferents, the combination of:

means into said first reaction chamber; p second scrubber means capableof scrubbing both nitric oxide and interferents from an air stream sothat an air stream with nitric oxide and interferents removed is emittedfrom the second scrubber means as an effluent;

means for introducing an air stream into said second scrubber means;

a second reaction chamber containing means for converting oxygen intoozone;

means for conveying the effluent from said second scrubber means throughsaid second reaction chamber into said first reaction chamber;

third scrubber means capable of scrubbing ozone in a gas streamcontaining nitrogen dioxide without appreciably affecting the nitrogendioxide content of the gas stream;

analyzer means for determining the nitrogen dioxide content of said gasstream; and

means for conveying the effluent from said first reaction chamberthrough said third scrubber means to said analyzer means.

23. An apparatus as set forth in claim 22 wherein said third scrubbermeans contains AgO as a scrubber material.

24. An apparatus as set forth in claim 23 wherein said first scrubbermeans contains a material selected from the group consisting of Ca(Ol-l)and KOH as a scrubber material.

2. A method as set forth in claim 1 wherein: the nitric oxide in saidgas stream is reacted with an excess amount of ozone so that saidresulting gas stream will contain ozone as well as nitrogen dioxide. 3.A method as set forth in claim 1 wherein: said ozone is produced byreacting ultraviolet radiation with an oxygen bearing gas.
 4. A methodas set forth in claim 1 wherein: said ozone is produced by generating anelectric discharge in an oxygen bearing gas.
 5. A method as set forth inclaim 1 wherein: said scrubber means contains AgO.
 6. A method as setforth in claim 1 wherein said sample gas stream contains interferentsother than ozone and including the additional steps of: providing secondscrubber means capable of scrubbing said interferents from said samplegas stream without appreciably affecting the nitric oxide contentthereof; and passing said sample gas stream through said second scrubbermeans prior to reacting the stoichiometric amount of ozone with thenitric oxide in the sample gas stream.
 7. A method as set forth in claim6 wherein said second scrubber means contains a material selected fromthe group consisting of Ca(OH)2 and KOH.
 8. A method as set forth inclaim 6 wherein said first-mentioned scrubber means contains AgO.
 9. Amethod as set forth in claim 6 wherein said first-mentioned scrubbermeans scrubs ozone by thermal decomposition thereof.
 10. A method ofcontinuously monitoring the nitric oxide content of an air sample streamcontaining interferents comprising the steps of: scrubbing theinterferents from the air sample stream; reacting the nitric oxide inthe air sample stream with an excess amount of ozone so as to completelyconvert the nitric oxide to nitrogen dioxide and also provide ozone inthe resulting gas stream; scrubbing the ozone from said resulting gasstream; and thereafter determining the nitrogen dioxide content of thescrubbed gas stream as a measure of the nitric oxide content of said airsample stream.
 11. A method as set forth in claim 10 wherein said ozoneis scrubbed from said resulting gas stream by passing the stream overAgO.
 12. A method as set forth in claim 11 wherein: said interferentsare scrubbed by passing the air sample stream over a material selectedfrom the group consisting of Ca(OH)2 and KOH.
 13. A method as set forthin claim 11 wherein said excess amount of ozone is provided by: passinga second air stream through an activated charcoal scrubber so thatinterferents are absorbed and a mixture consisting essentially ofnitrogen and oxygen is emitted from the scrubber as an effluent; andreacting the oxygen bearing effluent from the activated charcoalscrubber with ultraviolet radiation.
 14. A method of determining thetotal nitric oxide and nitrogen dioxide content of a sample gas streamcontaining interferents comprising the steps of: reacting the nitricoxide in the sample gas stream with at least the stoichiometric amountof ozone so as to completely convert the nitric oxide into nitrogendioxide; passing the resulting gas stream through a scrubber containingAgO to scrub said interferents and any ozone present in the gas stream;and thereafter determining the nitrogen dioxide content of the effluentemanating from said scrubber whereby the total nitric oxide and nitrogendioxide content of the sample gas stream is determined.
 15. An apparatusfor determining the nitric oxide content of a sample gas streamcomprising: means for providing a supply of ozone; a reaction chamber;means for conveying ozone from said first-mentioned means and the samplegas stream to said reaction chamber whereby the nitric oxide in saidsample gas stream will be converted into nitrogen dioxide in the chamberso that a nitrogen dioxide and ozone containing gas stream is emittedfrom the reaction chamber as an effluent; scrubber means for scrubbingozone from said gas stream witHout appreciably affecting the nitrogendioxide content of the gas stream so that a nitrogen dioxide containinggas stream is emitted from the scrubber means as an effluent; means forconveying the effluent from said reaction chamber to said scrubbermeans; analyzer means for determining the nitrogen dioxide content ofsaid gas stream; and means for conveying the effluent from said scrubbermeans to said analyzer means.
 16. An apparatus as set forth in claim 15wherein said scrubber means contains AgO as a scrubber material.
 17. Anapparatus as set forth in claim 15 wherein said scrubber means containsmeans capable of thermally decomposing ozone thereby scrubbing the ozonefrom said gas stream.
 18. An apparatus as set forth in claim 15 whereinsaid means for providing a supply of ozone includes a source ofultraviolet radiation for photo-oxidizing oxygen to ozone.
 19. Anapparatus as set forth in claim 15 wherein said means for providing asupply of ozone includes means for generating an electric dischargesufficient to convert oxygen into ozone.
 20. An apparatus as set forthin claim 15 including: second scrubber means capable of scrubbinginterferents from the sample gas stream without appreciably affectingthe nitric oxide content thereof so that a nitric oxide containing gasstream is emitted from the second scrubber means as an effluent; andsaid sample gas conveying means includes means for introducing thesample gas stream into said second scrubber means and means fordelivering the effluent from said second scrubber means to said reactionchamber.
 21. An apparatus as set forth in claim 20 wherein said secondscrubber means contains a material selected from the group consisting ofCa(OH)2 and KOH as a scrubber material.
 22. In an apparatus forcontinuously monitoring the nitric oxide content of a sample air streamcontaining interferents, the combination of: first scrubber meanscapable of scrubbing interferents from an air stream containing nitricoxide without appreciably affecting the nitric oxide content of thestream so that a nitric oxide containing gas stream is emitted from thefirst scrubber means as an effluent; means for introducing the sampleair stream into said first scrubber means; a first reaction chamberwherein nitric oxide in the sample air stream is converted to nitrogendioxide so that an air stream containing nitrogen dioxide is emittedfrom the first reaction chamber as an effluent; means for conveying theeffluent from said first scrubber means into said first reactionchamber; second scrubber means capable of scrubbing both nitric oxideand interferents from an air stream so that an air stream with nitricoxide and interferents removed is emitted from the second scrubber meansas an effluent; means for introducing an air stream into said secondscrubber means; a second reaction chamber containing means forconverting oxygen into ozone; means for conveying the effluent from saidsecond scrubber means through said second reaction chamber into saidfirst reaction chamber; third scrubber means capable of scrubbing ozonein a gas stream containing nitrogen dioxide without appreciablyaffecting the nitrogen dioxide content of the gas stream; analyzer meansfor determining the nitrogen dioxide content of said gas stream; andmeans for conveying the effluent from said first reaction chamberthrough said third scrubber means to said analyzer means.
 23. Anapparatus as set forth in claim 22 wherein said third scrubber meanscontains AgO as a scrubber material.
 24. An apparatus as set forth inclaim 23 wherein said first scrubber means contains a material selectedfrom the group consisting of Ca(OH)2 and KOH as a scrubber material.